INTERNET-DRAFT A. Vaha-Sipila Expires 24-Feb-2000 Nokia 19-Aug-1999 URLs for Telephone Calls Status of This Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. This document is an Internet-Draft. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. To view the entire list of current Internet-Drafts, please check the "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast). The distribution of this document before its expiry date is unlimited. Abstract This document specifies URL (Uniform Resource Locator) schemes terminal in the phone network and the connection types (modes of operation) that can be used to connect to that entity. This specification covers voice calls (normal phone calls, answering machines and voice messaging systems), facsimile (telefax) calls and data calls, both for POTS and digital/mobile subscribers. A. Vaha-Sipila Expires February, 2000 FORMFEED[Page 1] Internet-Draft URLs for Telephone Calls August 1999 Table of Contents A. Vaha-Sipila Expires February, 2000 FORMFEED[Page 2] Internet-Draft URLs for Telephone Calls August 1999 1. Introduction 1.1 New URL schemes URLs that designate phone or fax numbers that can be dialed have been brought forward in other Internet-Drafts. However, none of these has reached the RFC status. This document tries to remedy the situation. All interested parties are invited to submit comments on this Internet-Draft. Contact information can be found at the end of this document. See also [CONV-URL] for more discussion on conversational URLs. This specification defines three new URL schemes: "tel", "fax" and "modem". They are intended for describing a terminal that can be contacted using the telephone network. The description includes the subscriber (telephone) number of the terminal and the necessary parameters to be able to successfully connect to that terminal. The "tel" scheme describes a connection to a terminal that handles normal voice telephone calls, a voice mailbox or another voice messaging system or a service that can be operated using DTMF codes. The "fax" scheme describes a connection to a terminal that can handle telefaxes (facsimiles). The name (scheme specifier) for the URL is "fax" as recommended by [E.123]. The "modem" scheme describes a connection to a terminal that can handle incoming data calls. The term "modem" refers to a device that does digital-to-analog and analog-to-digital conversions; in addition to these, a "modem" scheme can describe a fully digital connection. The notation for phone numbers is the same which is specified in [RFC2303] and [RFC2304]. However, the syntax definition is a bit different due to the fact that this document specifies URLs whereas [RFC2303] and [RFC2304] specify electronic mail addresses. For example, "/" (used in URLs to separate parts in a hierarchical URL [RFC2396]) has been replaced by ";". 1.2 Formal definitions Formal definitions follow [RFC2234]. This specification uses elements from the 'core' definitions (Appendix A of [RFC2234]). 1.3 Requirements A. Vaha-Sipila Expires February, 2000 FORMFEED[Page 3] Internet-Draft URLs for Telephone Calls August 1999 Compliant software MUST follow this specification. Requirements are indicated by capitalized words as specified in [RFC2119]. 2. URL schemes for telephone calls 2.1 Applicability In this document, "user agent" means software that can detect and parse one or more of these URLs and place a call to the remote terminal using hardware and software at its disposal after it has been properly configured. These URL schemes are used to direct the user agent to place a call using the telephone network. The network in question may be a landline or mobile phone network. If the phone network differentiates between (for example) voice and data calls, or if the user agent has several different telecommunications equipment at its disposal, it is possible to specify which kind of call (voice/fax/data) is requested. The URL can also contain information about the capabilities of the remote entity, so that the connection can be established successfully. None of the URL schemes do have a 'path' in them - they are always absolute. The URLs are always case-insensitive. 2.2 "tel" URL scheme The URL syntax is formally described as follows. For the basis of this syntax, see [RFC2303]. telephone-url = telephone-scheme ":" telephone-subscriber telephone-scheme = "tel" telephone-subscriber = global-phone-number / local-phone-number global-phone-number = "+" 1*phonedigit [isdn-subaddress] [post-dial] *(area-specifier / future-extension) local-phone-number = 1*(phonedigit / dtmf-digit / pause-character) [isdn-subaddress] [post-dial] *(area-specifier / future-extension) isdn-subaddress = ";isub=" 1*phonedigit post-dial = ";postd=" 1*(phonedigit / dtmf-digit / pause-character) area-specifier = ";area=" "+" 1*phonedigit future-extension = ";" 1*(ALPHA / DIGIT) ["=" 1*(ALPHA / DIGIT) ["?" 1*(ALPHA / DIGIT) ]] A. Vaha-Sipila Expires February, 2000 FORMFEED[Page 4] Internet-Draft URLs for Telephone Calls August 1999 phonedigit = DIGIT / visual-separator visual-separator = "-" / "." / "(" / ")" pause-character = one-second-pause / wait-for-dial-tone one-second-pause = "p" wait-for-dial-tone = "w" dtmf-digit = "*" / "#" / "A" / "B" / "C" / "D" 2.3 "fax" URL scheme The URL syntax is formally described as follows (the definition reuses nonterminals from the above definition). For the basis of this syntax, see [RFC2303] and [RFC2304]. fax-url = fax-scheme ":" fax-subscriber fax-scheme = "fax" fax-subscriber = fax-global-phone / fax-local-phone fax-global-phone = "+" 1*phonedigit [isdn-subaddress] [t33-subaddress] [post-dial] *(area-specifier / future-extension) fax-local-phone = 1*(phonedigit / dtmf-digit / pause-character) [isdn-subaddress] [t33-subaddress] [post-dial] *(area-specifier / future-extension) t33-subaddress = ";tsub=" 1*phonedigit 2.4 "modem" URL scheme The URL syntax is formally described as follows (the definition reuses nonterminals from the above definitions). For the basis of this syntax, see [RFC2303]. modem-url = modem-scheme ":" remote-host modem-scheme = "modem" remote-host = telephone-subscriber *modem-params modem-params = ";type=" data-capabilities data-capabilities = accepted-modem ["?" data-bits parity stop-bits] accepted-modem = "V21" / "V22" / "V22b" / "V23" / "V26t" / "V32" / "V32b" / "V34" / "V90" / "V110" / "V120" / "B103" / "B212" / "X75" / "vnd." vendor-name "." modem-type data-bits = "7" / "8" parity = "n" / "e" / "o" / "m" / "s" stop-bits = "1" / "2" A. Vaha-Sipila Expires February, 2000 FORMFEED[Page 5] Internet-Draft URLs for Telephone Calls August 1999 vendor-name = 1*(ALPHA / DIGIT / "-" / "+") modem-type = 1*(ALPHA / DIGIT / "-" / "+") 2.5 Parsing telephone, fax and modem URLs 2.5.1 Call type The type of call is specified by the scheme specifier. "Tel" means that a voice call is opened. "Fax" indicates that the call should be a facsimile (telefax) call. "Modem" means that it should be a data call. Not all networks differentiate between the types of call; in this case, the scheme specifier indicates the telecommunications equipment type to use. 2.5.2 Phone numbers and their scope and indicate the phone number to be dialed. The phone number can be written in either international or local notation. All phone numbers SHOULD always be written in the international form if there is no good reason to use the local form. If is present, it defines the numbering area within which this number is valid. For example, if "area=+358", the given number (either a global or local number) is meaningful only within Finland, and if "area=+12345", the given number is only meaningful if the caller's global telephone number starts with "+12345". If is present, the user agent MUST NOT call out if the user agent is not located within that numbering area. Any telephone number MUST contain at least one or , that is, subscriber numbers consisting only of pause characters are not allowed. International numbers MUST begin with the "+" character. Local numbers MUST NOT contain that character. International numbers MUST be written with the country (CC) and national (NSN) numbers as specified in [E.123] and [E.164]. International numbers have the property of being totally unambiguous everywhere in the world if the user agent is properly configured. Local numbers MAY be used if the number only works from inside a certain geographical area or a network. Note that some numbers may work from several networks but not from the whole world - these SHOULD be written in international form. URLs containing local phone numbers should only appear in an environment where all user agents can get the call successfully set up by passing the number to the A. Vaha-Sipila Expires February, 2000 FORMFEED[Page 6] Internet-Draft URLs for Telephone Calls August 1999 dialing entity "as is". An example could be a company intranet, where all user agents are located under a the same private telephone exchange. If local phone numbers are used, the document in which they are present SHOULD contain an indication of the context in which they are intended to be used. In some regions, it is popular to write phone numbers using alphabetic characters which correspond to certain numbers on the telephone keypad. Letters in characters do not have anything to do with this, nor is this method supported by these URL schemes. It should also be noted that implementations MUST NOT assume that telephone numbers have a maximum, minimum or fixed length, or that they would always begin with a certain number. Implementors are encouraged to familiarize themselves with the international standards for telephone number notation. 2.5.3 Separators in phone numbers All characters MUST be removed from the phone number by the user agent before using it do dial out. These characters are present only to aid readability: they MUST NOT have any other meaning. Note that although [E.123] recommends the use of space (SP) characters as the separators, spaces MUST NOT be used in these URLs. 2.5.4 Converting the number to the local numbering scheme After the telephone number has been extracted, it is converted to the format that the user agent can use to place the call. (For example, the "+" character might be replaced by the international call prefix, or the international and trunk prefixes might be removed to place a local call.) Numbers that have been specified using or MUST be used by the user agent "as is", without any conversions. 2.5.5 Sending post-dial sequence after call setup The number may contain a sequence, which MUST be dialled using Dual Tone Multifrequency (DTMF) in-band signalling or pulse dialing after the call setup is complete. If the user agent does not support DTMF or pulse dialing after the call has been set up, MUST be ignored. In that case, the user SHOULD be notified. 2.5.6 Pauses in dialing and post-dial sequence A. Vaha-Sipila Expires February, 2000 FORMFEED[Page 7] Internet-Draft URLs for Telephone Calls August 1999 A local phone number or a post-dial sequence may contain characters which indicate a pause while dialing ("p"), or a wait for dial tone ("w"). User agents MAY support this method of dialing, and the final interpretation of these characters is left to the user agent. If it is not supported, user agents MUST ignore everything in the dial string after the first and the user SHOULD be notified. The user or the user agent MAY opt not to place a call if this feature is not supported and these characters are present in the URL. Any characters and all dial string characters after the first or SHOULD be sent to line using DTMF (Dual Tone Multifrequency) in-band signaling, even if dialing is done using direct network signaling (a digital subscriber loop or a mobile phone). If the local infrastructure does not support DTMF codes, the user agent MAY opt to use pulse dialing. However, it should be noted that certain services which are controlled using DTMF tones cannot be controlled with pulse dialing. 2.5.7 ISDN subaddresses A phone number MAY also contain an which indicates an ISDN subaddress. User agent SHOULD support ISDN subaddresses. These addresses are sent to the network by using a method available to the user agent (typically, ISDN subscribers send the address with the call setup signalling). If ISDN subaddressing is not supported by the caller, MUST be ignored and the user SHOULD be notified. The user or the user agent MAY opt not to place a call if this feature is not supported. 2.5.8 T.33 subaddresses A fax number MAY also contain a , which indicates the start of a T.33 subaddress [T.33]. User agents SHOULD support this. Otherwise MUST be ignored and the user SHOULD be notified. The user or the user agent MAY opt not to place a call if this feature is not supported. 2.5.9 Data call parameters indicate the minimum compliance required from the user agent to be able to connect to the remote entity. The minimum compliance is defined as being equal to or a superset of the A. Vaha-Sipila Expires February, 2000 FORMFEED[Page 8] Internet-Draft URLs for Telephone Calls August 1999 capabilities of the listed modem type. The user agent MUST call out using compatible hardware, or request that the network provides such a service. For example, if the user agent only has access to a V.22bis modem and the URL indicates that the minimum acceptable connection is V.32bis, the user agent MUST NOT try to connect to the remote host since V.22bis is a subset of V.32bis. However, if the URL lists V.32 as the minimum acceptable connection, the user agent can use V.32bis to create a connection since V.32bis is a superset of V.32. This feature is present because modem pools often have separate numbers for slow modems and fast modems, or have different numbers for analog and ISDN connections, or may use proprietary modems that are incompatible with standards. It is somewhat analogous to the connection type specifier (typecode) in FTP URLs [RFC1738]: it provides the user agent with information that can not be deduced from the scheme specifier, but is helpful for successful operation. This also means that the number of data and stop bits and parity MUST be set according to the information given in the URL, or to default values given in this document, if the information is not present. The capability tokens are listed below. If capabilities suggest that it is impossible to create a connection, the connection MUST NOT be created. If new modem types are standardized by ITU-T, this list can be extended with those capability tokens. Tokens are formed by taking the number of the standard and joining together the first letter (for example, "V"), number (for example, 22) and the first letter of the postfix (for example "bis" would become "b"). Proprietary modem types MUST be specified using the 'vendor naming tree', which takes the form "vnd.x.y", in which "x" is the name of the entity from which the specifications for the modem type can be acquired and "y" is the type or model of the modem. Vendor names MUST share the same name space with vendor names used in MIME types [RFC2048]. Submitting the modem types to ietf-types list for review is strongly recommended. New capabilities MUST always be documented in an RFC, and they MUST refer to this document or a newer version of it. Capability Explanation A. Vaha-Sipila Expires February, 2000 FORMFEED[Page 9] Internet-Draft URLs for Telephone Calls August 1999 V21 ITU-T V.21 V22 ITU-T V.22 V22b ITU-T V.22bis V23 ITU-T V.23 V26t ITU-T V.26ter V32 ITU-T V.32 V32b ITU-T V.32bis V34 ITU-T V.34 V90 ITU-T V.90 V110 ITU-T V.110 V120 ITU-T V.120 X75 ITU-T X.75 B103 Bell 103 B212 Bell 212 Data bits: "8" or "7" The number of data bits. If not specified, defaults to "8". Parity: "n", "e", "o", Parity. None, even, odd, mark or "m", "s" space parity, respectively. If not specified, defaults to "n". Stop bits: "1" or "2" The number of stop bits. If not specified, defaults to "1". 2.5.10 Additional parameters In addition to T.33 and ISDN subaddresses, modem types and area specifiers, future extensions to this URL scheme may add other additional parameters ( in the BNF) to these URLs. These parameters are added to the URL after a semicolon (";"). Implementations MUST be prepared to handle additional and/or unknown parameters gracefully. Implementations MAY opt not to use the URL if it contains unknown parameters. 2.6 Examples of Use tel:+358-555-1234567 This URL instructs the user agent to place a voice call to the specified number in Finland. The hyphens are included to make the number more human-readable: country and area codes have been separated from the subscriber number. fax:+358.555.1234567 The above URL instructs the user agent to place a fax call to the specified number. It uses dots instead of hyphens as separators, but they have no effect on the functionality. A. Vaha-Sipila Expires February, 2000 FORMFEED[Page 10] Internet-Draft URLs for Telephone Calls August 1999 modem:+3585551234567;type=v32b?7e1;type=v110 This URL instructs the user agent to place a data call to the specified number. The user agent may opt to use either a ITU-T V.32bis modem (or a faster one, which is compatible with V.32bis), using settings of 7 data bits, even parity and one stop bit, or an ISDN connection using ITU-T V.110 protocol. tel:+358-555-1234567;postd=pp22 The above URL instructs the user agent to place a voice call to +358-555-1234567, then wait for an implementation-dependent time (for example, two seconds) and emit two DTMF dialing tones "2" on the line (for example, to choose a particular extension number, or to invoke a particular service). tel:0w003585551234567 This URL places a voice call to the given number. The number format is intended for local use: the first zero opens an outside line, the "w" character waits for a second dial tone, and the number already has the international access code appended to it ("00"). This kind of phone number MUST NOT be used in an environment where all users of this URL might not be able to successfully dial out by using this number directly. However, this might be appropriate for pages in a company intranet. 2.7 Rationale behind the syntax 2.7.1 Why distinguish between call types? URLs locate resources, which in this case is some telecommunications equipment at a given phone number. However, it is not necessarily enough to know the subscriber number in order to successfully communicate with that equipment. Digital phone networks distinguish between voice, fax and data calls (and possibly other types of calls, not discussed in this specification). To be able to successfully connect to, say, a fax machine, the caller may have to specify that a fax call is being made. Otherwise the call might be routed to the voice number of the subscriber. In this sense, the call type is an integral part of the 'location' of the target resource. The reason to have the call type in the scheme specifier is to make the URL simple to remember and use. Making it a parameter, much like the way modem parameters are handled now, will substantially reduce the usability of this URL (to the humans). A. Vaha-Sipila Expires February, 2000 FORMFEED[Page 11] Internet-Draft URLs for Telephone Calls August 1999 2.7.2 Why "tel" is "tel"? There has been discussion on whether the scheme name "tel" is appropriate. To summarize, these are the points made against the other proposals. callto URL schemes locate a resource and do not specify an action to be taken. telephone Too long. Also, "tel" considered to be a more international form. phone Was countered on the basis that "tel" is more internationally acceptable. 2.7.3 Why to use E.164 numbering? It should be noted that phone numbers may have 'hierarchical' characteristics, so that one could build a 'forest' of phone numbers with country codes as roots, area codes as branches and subscriber numbers as leaves. However, this is not always the case. Not all areas have area codes; some areas may have different area codes depending on how one wants to route the call; some numbers must always be dialled "as is", without prepending area or country codes; and area codes can and do change. Usually, if something has a hierarchical structure, the URL syntax should reflect that fact. These URLs are an exception. Phone numbers are written almost always in some form which resembles the E.164 notation. Because of this, the syntax in this specification is intuitively clear to most people. This is the usual way to write phone numbers in business cards, advertisements, telephone books and so on. Also, when writing the phone number in the form described in this specification, the writer does not need to know which part of the number is the country code and which part is the area code. If a hierarchical URL would be used (with a "/" character separating the parts of the phone numbers), the writer of the URL would have to know which parts are which. Finally, when phone numbers are written in the international form as specified here, they are unambiguous and can always be converted to the local dialing convention, given that the user agent has the knowledge of the local country and area codes. 2.7.4 Not everyone has the same equipment as you A. Vaha-Sipila Expires February, 2000 FORMFEED[Page 12] Internet-Draft URLs for Telephone Calls August 1999 There are several ways for the subscriber to dial a phone number: - By pulse dialing. Typically old telephone exchanges. Usually this dialing method has only to be used to set up the call; after connecting to the remote entity, can be sent to the line using DTMF, because it will typically be processed by the remote entity, not the telephone network. - By DTMF. These are the 'beeps' that you hear when you dial on most phones. - By direct network signalling. ISDN subscribers and mobile phone users usually have this. There is no dial tone (or if there is, it is generated locally by the equipment), and the number of the called party is communicated to the telephone network using some network signalling method. After setting up the call, sequences are usually sent using DTMF codes. 3. Comments on usage These are examples of the recommended usage of this URL in HTML documents. First of all, the number SHOULD be visible to the end user, if it is conceivable that the user might not have a user agent which is able to use these URLs. Telephone: +358-555-1234567 Second, on a public HTML page, the telehone number in the URL SHOULD always be in the international form, even if the text of the link uses some local format. Telephone: (0555) 1234567 or even For more info, call 1-555-IETF- RULZ-OK. Moreover, if the number is a , and the scope of the number is not clear from the context in which the URL is displayed, a human-readable explanation SHOULD be included. For customer service, dial 1234 (only from Terrific Telecom mobile phones). A. Vaha-Sipila Expires February, 2000 FORMFEED[Page 13] Internet-Draft URLs for Telephone Calls August 1999 4. References NOTE. References to Internet-Drafts will be removed from the final document which will be submitted to the RFC-Editor. [CONV-URL] Conversational Multimedia URLs. 1997. Pete Cordell. An Internet-Draft (work in progress). [RFC1738] Uniform Resource Locators (URL). December 1994. T. Berners-Lee et al. RFC 1738. [RFC1866] Hypertext Markup Language - 2.0. November 1995. T. Berners-Lee & D. Connolly. RFC 1866. [RFC2048] Multipurpose Internet Mail Extensions (MIME) Part Four: Registration Procedures. November 1996. N. Freed et al. RFC 2048. [RFC2119] Key Words for Use in RFCs to Indicate Requirement Levels. March 1997. S. Bradner. RFC 2119. [RFC2234] Augmented BNF for Syntax Specifications: ABNF. November 1997. D. Crocker et al. RFC 2234. [RFC2303] Minimal PSTN Address Format in Internet Mail. March 1998. C. Allocchio. RFC 2303. [RFC2304] Minimal FAX Address Format in Internet Mail. March 1998. C. Allocchio. RFC 2304. [RFC2396] Uniform Resource Identifiers (URI): Generic Syntax. August 1998. T. Berners-Lee et al. RFC 2396. [E.123] ITU-T Recommendation E.123: Telephone Network and ISDN Operation, Numbering, Routing and Mobile Service: Notation for National and International Telephone Numbers. 1993. [E.164] ITU-T Recommendation E.164: Telephone Network and ISDN Operation, Numbering, Routing and Mobile Service: Numbering Plan for A. Vaha-Sipila Expires February, 2000 FORMFEED[Page 14] Internet-Draft URLs for Telephone Calls August 1999 the ISDN Era. 1991. [T.33] ITU-T Recommendation T.33: Facsimile Routing Utilizing the Subaddress. 1996. 5. Security Considerations It should be noted that the user agent SHOULD NOT call out without the knowledge of the user because of associated risks, which include - call costs (including long calls, long distance calls, international calls and premium rate calls) - wrong numbers inserted on web pages by malicious users - making the user's phone line unavailable (off-hook) for a malicious purpose - opening a data call to a remote host, thus possibly opening a back door to the user's computer - revealing the user's (possibly unlisted) phone number to the remote host in the caller identification data All of these risks MUST be taken into consideration when designing the user agent. The user agent SHOULD have some mechanism that the user can use to filter out unwanted numbers. The user agent SHOULD NOT use rapid redialing of the number if it is busy to avoid the congestion of the (signaling) network. Also, the user agent SHOULD detect if the number is unavailable or if the call is terminated before the dialing string has been completely processed (for example, the call is terminated while waiting for user input) and not try to call again, unless instructed by the user. 6. Acknowledgements Contributors include numerous people from IETF FAX, PINT, URI and URLREG mailing lists, as well as from World Wide Web Consortium and several companies, plus several individuals. Thanks to all people who offered criticism, corrections and feedback. All phone numbers and company names used in the examples of this specification are fictional. Any similarities to real entities are coincidental. A. Vaha-Sipila Expires February, 2000 FORMFEED[Page 15] Internet-Draft URLs for Telephone Calls August 1999 7. Authors' Addresses Contact person and version control responsibility for this specification: Nokia Mobile Phones Antti Vaha-Sipila P. O. Box 68 FIN-33721 Tampere Finland Electronic mail: avs@iki.fi antti.vaha-sipila@nokia.com Please include your name and electronic mail address in all communications. If you want to receive the newest version of this specification electronically, send mail to the address above. This document expires on the 24th of February, 2000, or when a new version is released. 8. Full Copyright Statement To be added to the final RFC. A. Vaha-Sipila Expires February, 2000 FORMFEED[Page 16] Internet-Draft URLs for Telephone Calls August 1999 1. Introduction ................................................ 3 1.1 New URL schemes ............................................ 3 1.2 Formal definitions ......................................... 3 1.3 Requirements ............................................... 3 2. URL schemes for telephone calls ............................. 4 2.1 Applicability .............................................. 4 2.2 "tel" URL scheme ........................................... 4 2.3 "fax" URL scheme ............................................................. 5 2.4 "modem" URL scheme ............................................................. 5 2.5 Parsing telephone, fax and modem URLs ............................................................. 6 2.5.1 Call type ............................................................. 6 2.5.2 Phone numbers and their scope ............................ 6 2.5.3 Separators in phone numbers .............................. 7 2.5.4 Converting the number to the local numbering scheme ...... 7 2.5.5 Sending post-dial sequence after call setup .............. 7 2.5.6 Pauses in dialing and post-dial sequence ................. 7 2.5.7 ISDN subaddresses ........................................ 8 2.5.8 T.33 subaddresses ........................................ 8 2.5.9 Data call parameters ..................................... 8 2.5.10 Additional parameters ............................................................. 10 A. Vaha-Sipila Expires February, 2000 FORMFEED[Page 17] Internet-Draft URLs for Telephone Calls August 1999 2.6 Examples of Use ............................................ 10 2.7 Rationale behind the syntax ................................ 11 2.7.1 Why distinguish between call types? ..................... 11 2.7.2 Why "tel" is "tel"? ..................................... 12 2.7.3 Why to use E.164 numbering? ............................................................. 12 2.7.4 Not everyone has the same equipment as you ............... 12 3. Comments on usage ........................................ 13 4. References ............................................... 14 5. Security Considerations ..................................... 15 6. Acknowledgements ............................................ 15 7. Authors' Addresses .......................................... 16 8. Full Copyright Statement .................................... 16 A. Vaha-Sipila Expires February, 2000 FORMFEED[Page 18]